1. Field of the Invention
The invention relates to a process for an automatic adjustment of a layered substrate and more particularly the adjustment of a semiconductor wafer by the use of adjustment marks with an optical alignment technique. In particular, the present invention is directed to an improvement in the method disclosed and claimed in the U.S. patent application of Dietrich Widmann and Johann Binder, U.S. Ser. No. 709,129, filed July 26, 1976 whose disclosure is hereby incorporated by reference thereto.
2. Prior Art
In semiconductor technology, it is known to produce structures on a semiconductor surface by a method which includes regional or zonal exposure of a portion of the photo resist layer disposed on the semiconductor surface, development of the exposed photo resist layer, and then etching the regions of the surface no longer covered by the photo resist layer after development to produce the desired surface structure. The exposure of the regions or zones of the photo resist layer is carried out either with the aid of a contact printing process, wherein the exposure is through a photo mask (or photography mask) which is pressed onto the layer of photo resist which is carried on the suface of the semiconductor substrate or is accomplished with the aid of a projection printing process wherein a photo mask is projected on the photo resist layer by means of high resolution objective or compound lens arrangement.
With a projection printing process, the photo mask can be projected onto the photo resist layer either true to scale or in a reduced fashion. In the case of a 1:10 reduction during the projection of the photo mask, a resolution capacity of approximately 500 lines per mm and adjustment precision of .+-. 0.2 .mu.m is obtained. In contrast with the contact printing process, the advantages of projection printing process particularly lie in the fact that the mask and the semiconductor disk are free of mechanical stresses and that the mask can be readily controlled as to its precision. In addition, expansions, bendings or other deformations of the semiconductor disk or substrate have only a minimal effect on the precise location of successive structural planes. However, a disadvantage does exist in that the exposure of the entire semiconductor disk having, for example, a diameter of 50 mm is very costly due to the requirement of a large mask and a very large objective or lens system.
Accordingly, it is more economical to use a mask and lens system which only exposes one portion or zone of the layer of photo resist on the semiconductor surface and then expose additional zones or regions of the photo resist after additional adjustments, focusing and exposure operations. However, this method can only be economically utilized if the adjustment, focusing and exposure can be automatically achieved in the case of a reductive projection.
In the step and repeat apparatus employed for this purpose, the distance between the objective utilized for reproduction and the surface of the photo resist layer disposed on the semiconductor disk is either mechanically adjusted or regulated to a precision of .+-. 0.2 .mu.m via an air cushion. The relative position between the semiconductor disk and the objective in the direction of the optical axis is usually not changed or adjusted between each exposure of the zones of the semiconductor disk. For this reason, uneven surfaces in the semiconductor disk will produce unsharp reproductions of the image of the photo mask on the photo resist layer if these uneven locations exceed the amount of the depth of focus of the objective, which, for example, during a reduction of 1:10 constitutes .+-.2 .mu.m. In addition, mechanical influences and temperature changes can also cause changes in the length or longitudinal deformation which, due to the lack of a distance adjustment between the objective and the surface of the semiconductor disk, will likewise lead to an unsharp, blurred or undefined reproduction of the image of the mask. If these losses in the definition of the image, which losses are due to unsharp or blurred reproduction of the image, are to be avoided, the reproduction exposure apparatus must be focused manually before each exposure of an area or zone of the substrate.